Purification process of carbon nanotubes

ABSTRACT

The present invention describes a new method for the purification of carbon nanotubes from contaminants deriving from industrial production, constituted by amorphous or carbon crystalline material and metals used for catalysis. The method provides for their organic furretionalization obtaining dirivatized nanotubes, precipitation of functionalized nanotubes and subsequent regeneration of non-functionalized nanotubes by heat treatment.

FIELD OF THE INVENTION

The present invention relates to a new method for the purification ofcarbon nanotubes through organic functionalization, precipitation andheat treatment and to the functionalized nanotubes.

STATE OF THE ART

Carbon nanotubes (NT) are carbon materials that are currently fabricatedfrom graphite through procedures generally known as electric arcdischarge, HIPCO (High-Pressure CO), laser-desorption, laser-ablation,plasma (PVD) or chemical vapor deposition (CVD). Following thesetreatments the initial material adopts a highly orderly structureconstituted by one wall (single-walled nanotubes SWNT) or several walls(multiple-walled nanotubes MWNT) with miniaturized cylindrical shapevarying in diameter and length according to the type of treatment, inwhich the carbon atoms combined together form a prevalently hexagonalhoneycomb pattern. This particular arrangement providers these materialswith unexpected and interesting chemico-physical and mechanicalproperties making them an extremely important material from theapplicative aspect. The carbon nanotubes are in fact highly resistant tovoltage which determines important electronic, optical and mechanicalproperties for the use of these materials in many fields of applicationas metal conductors or semi-conductors, insulating materials ormaterials with high mechanical strengths. They can thus be used inelectronic and opto-electronic equipment (electrical and electronicmicrocircuits, diodes, transistors, sensors, field emission displays,vacuum fluorescent displays or sources of white light), and alsopolymeric compound materials with high electrical, thermal andmechanical strength. However, all the procedures generally used forfabrication provide a raw product with a great number of impurities,differing according to the method used, which include inert carbonparticles, amorphous carbon, fullerenes and catalysis metals. Thesecontaminants constitute serious limits both to their study and to theirmany uses. Various approaches have been taken to purify them,essentially based on: oxidization processes with acids or mixtures ofacids (nitric and/or sulphuric, and/or hydrochloric acid) both insolution and gaseous, filtration, separation by centrifugation orchromatography. Whatever the approach used for purification, the firstproblem to be overcome is the question of their insolubility. For thisaspect the initial material, before actual purification, is subjected inmany cases to drastic dispersion processes in solvents or in water, suchas pulverization, sonication or ultrasonication with or withoutsurfactants. In any case these are purification processes that requireseveral phases, even at high temperatures, in which the oxidizingtreatment(s) with acids are essential and preferred to eliminate bothmetal and carbon contaminants. However, in this way the nanotubes arealso impaired, destroying a part of them and introducing numerousstructural defects with considerable influences on theirchemico-physical and mechanical properties.

Chiang I. W. et al. (J. Phys. Chem. B 2001, 105, 8297–8301) describes aSWNT purification method through a high temperature oxidization process,obtaining a yield of about 30% and a high degree of purity. Nonetheless,this purification method, in addition to obtaining low yield rates, hasthe disadvantage of also oxidizing the NT, with consequent modificationsin their molecular structure.

Therefore, as it is currently impossible to purify the nanotubes with ahigh degree of yield and purity at industrial level in a manner thatsafeguards them, they are usually marketed as they are produced.

In view of the fact that however they are fabricated, nanotubes areextremely costly materials and that for some industrial uses unshortenednanotubes without structural defects are preferred, the object of thepresent invention is to establish a purification method industriallyapplicable to the raw material, which has a good yield rate both asregards quantity and quality and which does not impair the structure ofthe nanotubes.

SUMMARY OF THE INVENTION

A new, non-destructive purification process for industrially producednanotubes (pristine nanotubes p-NT), both SWNT and MWNT, has now beenfound, and is the object of the present invention, characterized by atleast the following steps:

-   -   solubilization of the nanotubes (p-NT) through their organic        functionalization in which the functionalization reaction is        obtained on the nanotubes with 1,3-dipolar reaction of        azomethine ylides and separation from metal contaminants,    -   purification from carbon contaminants of functionalized        nanotubes (f-NT) obtained in the previous step by precipitation        with solvents from their organic solutions,    -   heat treatment of functionalized nanotubes obtained in the        previous step to regenerate the initial non-functionalized        carbon nanotubes free of metal and carbon contaminants.

A further object of the invention relates to new functionalized nanotubeobtained with the 1,3-dipolar reaction of functionalization azomethineylides according to the first step.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1: Transmission electron microscopy (TEM) images of pristinesingle-walled nanotube (p-SWNT);

FIG. 2: Transmission electron microscopy (TEM) image of functionalizedsingle-walled nanotube (f-SWNT);

FIG. 3: Transmission electron microscopy (TEM) images ofde-functionalized single-walled nanotube at the end of purificationprocess.

DETAILED DESCRIPTION OF THE INVENTION

Solubilization of nanotubes, both SWNT and MWNT, throughfunctionalization has been described by inventors starting out withmaterials prepared and purified according to conventional procedures(Georgakilas G. et. al. J. Am. Chem. Soc. 2002, 124, 760–761).

To attain the object of the present invention, the purification processinvolved in the present invention is characterized by the followingphases:

-   -   solubilization through organic functionalization of nanotubes as        produced industrially (pristine nanotubes p-NTY through        1,3-dipolar reaction of azomethine ylides and separation from        metal contaminants,    -   purification of functionalized nanotubes (f-NT) by precipitation        with solvents from organic solutions of functionalized        nanotubes,    -   heat treatment of functionalized, nanotubes (f-NT) to regenerate        the initial non-functionalized nanotubes.

When establishing the purification process described it was unexpectedlydiscovered that the functionalization reaction of the nanotubes can alsobe obtained when the initial material is a raw material containing metalcontaminants deriving from industrial production and that each of thephases required contributes towards purification of the initial rawproduct, but that only through a combination of these is a final productwith a high level of purity obtained.

In detail, the purification method established is as describedhereunder.

The industrial nanotubes (p-NT), represented by raw material producedaccording to any one of the production processes mentioned hereinbefore,are functionalized through a 1,3-dipolar reaction which involves thep-nanotubes and the following components:

a) compounds with a general formula 1R′—NH—CHR″—COOR′″  1

where R′, R″ and R′″ equal to or different from one another may be:

H or alkyl groups with the formula C_(n)H_(2n+1) with n between 1 and20, or an aromatic group, or limited to R′, R′″ an ether group with theformula (CH₂CH₂O)_(n) with n between 1 and 20 and

b) compounds with a general formula 2R″″—CHO   2

where R″″ may be H or an alkyl group with the formula C_(n)H_(2n+1) withn between 1 and 20, or an ether group with the formula (CH₂CH₂O)_(n)with n between 1 and 20, or an aromatic group.

The functionalization reaction is conducted in a dipolar aprotic solventchosen from the group constituted by dimethylformamide (DMF),dimethylsulphoxide, sulpholane, orthodichlorobenzene with reagents inexcess and temperature over 100° C. for at least 24 hours.

Any functionalized nanotubes obtained through the 1.3-dipolar reactionof azomethine ylides, being soluble, can be used in the purificationprocess of p-nanotubes. Furthermore, respect to the paper cited(Georgakilas G. et. al. 2002, ref.cit.), the functionalized nanotubesobtained through the reaction above mentioned are also new except whenR′, R″, R′″ and R″″ are equal to one another and equal to H; R′ is equalto an ether group with the formula —(CH₂CH₂O)₃—CH₃ and R″, R′″ and R″″are equal to one another and equal to H; R′ is equal to an alkyl groupwith the formula —CH₂(CH₂)₆CH₃ and R″, R′″ and R″″ are equal to oneanother and equal to H; R′ is equal to an ether group with the formula—(CH₂CH₂O)₃—CH₃, R″, R′″ are equal to each other and equal to H and R″″is equal to -phenyl-OCH₃; R′ is equal to an ether group with the formula—(CH₂CH₂O)₃—CH₃, R″, R′″ are equal to each other and equal to H and R″″is equal to a pyrene group.

Purely for non-limiting explanatory purposes of the present invention, atypical process for functionalization of commercial nanotubes isdescribed hereunder.

EXAMPLE 1 Organic Functionalization Reaction

100 mg of SWNT carbon nanotubes are suspended in 300 ml ofdimethylformamide (DMF). The mixture is heated to 140° C. and a solutionof N-octylglycine ethyl ester in DMF (500 mg in 10 ml) is added inportions of 2 ml every 24 hours with 500 mg of paraformaldehyde eachtime. Lastly, after 50 hours, 500 mg of N-octylglycine ethyl ester and2.5 g of aldehyde are added to the initial suspension. The mixture isheated for a further 72 hours.

EXAMPLE 2 Organic Functionalization Reaction

1.00 mg of SWNT carbon nanotubes are suspended in 300 ml ofdimethylformamide (DMF). The mixture is heated to 140° C. and a solutionof N-methylglycine in DMF (500 mg in 10 ml) is added in portions of 2 mlevery 24 hours with 500 mg of n-heptaldehyde each time. Lastly, after 50hours, 500 mg of N-methylglycine and 2.5 g of aldehyde are added to theinitial suspension. The mixture is heated for a further 72 hours.

In both examples the organic phase is separated by filtration on paperunder vacuum and the solution is transferred to a rotary evaporatorwhere the DMF is removed quantitatively. The remaining brown oilyresidue is diluted in 200 ml of dichloromethane and washed with water(5×200 ml). The organic phase is dried over Na₂SO₄ and after beingfiltered (to remove the Na₂SO₄) the solution is concentrated undervacuum. The residue is dissolved in 2 ml of dichloromethane andsubsequently 10 ml of methanol is added to the mixture. The solid partthat surfaces after adding the methanol is separated by centrifugationor filtering and washed repeatedly with methanol, until the alcoholphase no longer colours.

The products resulting from this reaction (functionalized nanotubesf-NT) have a high level of solubility in organic solvents, in which themetal particles present remain insoluble and are therefore eliminatedwith conventional mechanical means such as filtration under vacuum,centrifugaton, etc. The organic phase containing the f-NT isconcentrated to eliminate the reaction solvent and the residue isdissolved in polar or apolar solvents chosen fom the group constitutedby dichloromethane, chloroform, toluene, washed several times withwater. The organic phase is then dried and concentrated after removingthe drying agent. The functionalized nanotubes (f-NT) again solubilizedin polar or apolar solvents chosen from the group constituted bydichloromethane, chloroform, toluene are then precipitated with one ormore treatments with polar or apolar solvents chosen from the groupconstituted by diethylether, petroleum ether, alkanes, alcohols,separated with mechanical means (for example centrifugation orfiltration) and washed once or several times with the same solvent usedfor precipitation.

The precipitated nanotubes (f-NT) obtained are again dissolved withorganic solvents chosen from the group constituted by chloroform andmethylene chloride. The subsequent precipitation phase, as describedhereinbefore, is performed with polar or apolar organic solvents chosenfrom the group constituted by diethylether, petroleum ether, alkanes,alcohols, separated with mechanical means (for example centrifugation orfiltration). In this phase the contaminants constituted by amorphouscarbon materials remaining in solution are eliminated and thereforeprecipitations with solvents may be one or several according to thedegree of purity to be obtained. The precipitation phase can be repeatedseveral times on the nanotubes remaining in the solution until noprecipitates are obtained.

The purified f-NT deriving from the precipitation phase are thendefunctionalized by dry heating in an atmosphere with air, or inertgases, such as nitrogen or argon, or preferably under vacuum, attemperatures ranging from 250° to 350° C. for times between 1 minute andone hour. The initial nanotubes, without the contaminants fromindustrial production, are thus obtained after heat treatment withmechanical means chosen from centrifugation and filtration ofsuspensions obtained by sonication of these in polar or apolar solventschosen from the group constituted by dichloromethane, chloroform,toluene.

Purely for non-limiting explanatory purposes of the present invention, atypical purification process performed on functionalized nanotubes isdescribed hereunder.

EXAMPLE 3 Purification of Functionalized Nanotubes

80 mg of functionalized nanotubes according to example 1 and obtainedusing N—(CH₂CH₂OCH₂CH₂OCH₂CH₂OCH₃)-glycine with paraformaldehyde aredissolved in 300 ml of CHCl₃. In this way a transparent brown solutionis obtained. Diethylether is added in drops to the solution understirring at ambient temperature. This is added until a precipitateappears. The solution is filtered and diethylether is once again added.The precipitate is recovered from the paper filter dissolving it inCH₂Cl₂. After the CH₂Cl₂ has evaporated, the solid material is washedwith diethylether, centrifuged and dried under vacuum. This procedure isrepeated three times; three precipitates (P1, P2 e P3) are obtained.Moreover, after evaporation of the chloroform, the material that had notyet precipitated (S) is also obtained in solid state.

The precipitates P1, P2 and P3 appear respectively after the addition of480, 300 and 300 ml of diethylether, each of these precipitates is equalto about 10 mg; S is equal to 50 mg with a total yield of 100%.

The purity of each of these fractions is determined by TEM analysis.

EXAMPLE 4 Defunctionalization and Regeneration of Nanotubes

2 mg of functionalized nanotubes are placed in a metal (aluminium)capsule and heated to 300° C. under a flow of nitrogen for 30 minutes.The capsule is transferred to a conical flask containing 20 ml ofdichloromethane, where the content of the capsule is freed in suspensionin the organic phase through 10–20 sec of sonication in a normalultrasonic bath. The solid product is separated by centrifugation orfiltering, washed with 10 ml of chloroform and dried under vacuum for 2hours. At the end of this step 1.4 mg of purified nanotubes have beenobtained with a yield of 100%.

The results obtained with the process described are set down hereunder.

FIG. 1 shows trasmission electron microscopy (TEM) of the nanotubes(p-SWNT) before functionalization and precipitation containing highquantities of metal particles and carbon materials and FIG. 2 thoseobtained after functionalization (f-SWNT) with no traces of metalparticles and with no carbon materials.

FIG. 3 shows the TEM photographs obtained on the nanotubes afterprecipitation with solvents and heat treatment.

Moreover, it must be pointed out that the initial nanotubes (p-SWNT) andthose obtained at the end of the purification process after heattreatment (r-SWNT) were subjected to UV-Vis-NIR and IR-Raman analysis.The results show that the two types of nanotubes have the sameelectronic behaviour, confirming that the purification process has notcaused any structural damage to the nanotubes thus purified.

Substantially, by adopting the purification process described, the sameresults are obtained starting out with p-SWNT and with the p-MWNT type.Without departing from the scope of the invention it is possible forthose skilled in the art to make all the modifications and improvementsto the process described in and the object of the present inventionsuggested, by normal experience and development in the technique.

1. Purification process of carbon nanotubes characterized in that itcomprises at least the following steps, solubilization of the carbonnanotubes (p-NT) through their organic functionalization in which thefunctionalization reaction is obtained on the nanotubes with 1,3-dipolarreaction of azomethine ylides and separation from metal contaminants,purification from carbon contaminants of functionalized carbon nanotubes(f-NT) obtained in the previous step by precipitation of functionalizedcarbon nanotubes with solvents from their organic solutions, heattreatment of functionalized carbon nanotubes obtained in the previousstep to regenerate the initial non-functionalized carbon nanotubes freefrom traces of metal and carbon contaminants.
 2. Purification process ofcarbon nanotubes as claimed in claim 1 wherein the carbon nanotubes arefunctionalized through a 1,3-dipolar reaction with: a) compounds with ageneral formula 1R′—NH—CHR″—COOR′″  1 where R′, R″ and R′″ equal to or different from oneanother may be H or alkyl groups with the formula C_(n)H_(2n+1) with nbetween 1 and 20, or an aromatic group, or limited to R′, R′″ an ethergroup with the formula (CH₂CH₂O)_(n) with n between 1 and 20 and b)compounds with a general formula 2R″″—CHO  2 where R″″ may be H or an alkyl group with the formulaC_(n)H_(2n+1) with n between 1 and 20, or an ether group with theformula (CH₂CH₂O)_(n) with n between 1 and 20, or an aromatic group. 3.Purification process of carbon nanotubes as claimed in claim 2 whereinthe 1,3-dipolar reaction is conducted in dipolar aprotic solvent chosenfrom the group constituted by dimethylformamide (DMF),dimethylsulphoxide, sulpholane, orthodichlorobenzene with reagents inexcess and temperature over 100° C. for at least 24 hours. 4.Purification process of carbon nanotubes as claimed in claim 1 whereinthe separation from metal contaminants is obtained with mechanical meanschosen from filtration and centrifugation of the organic solutionderiving from the functionalization reaction of raw carbon nanotubes. 5.Purification process of carbon nanotubes as claimed in claim 1 whereinpurification from carbon contaminants is obtained starting out with anorganic solution containing functionalized carbon nanotubes in polar orapolar solvents, chosen from the group constituted by methylene chlorideand chloroform, by precipitation with one or more treatments with polaror apolar solvents, chosen from the group constituted by diethylether,petroleum ether, alkanes, alcohols and separation with mechanical meanschosen from centrifugation and filtration.
 6. Purification process ofcarbon nanotubes as claimed in claim 1 wherein the initial carbonnanotubes are obtained by dry heating the functionalized carbonnanotubes in an atmosphere with air or inert gases at temperaturesranging from 250° to 350° C. for times between 1 minute and one hour. 7.Purification process of carbon nanotubes as claimed in claim 1 whereinthe initial carbon nanotubes are obtained by dry heating thefunctionalized carbon nanotubes at temperatures ranging from 250° to350° C. for times between 1 minute and one hour under vacuum. 8.Purification process of carbon nanotubes as claimed in claim 1 whereinthe initial carbon nanotubes defunctionalized by heat treatment areseparated with mechanical means chosen from centrifugation andfiltration of their suspensions obtained by sonication in polar orapolar organic solvents chosen from the group constituted bydichloromethane, chloroform, toluene.
 9. Functionalized carbon nanotubesobtainable with 1,3-dipolar reaction with the carbon nanotubes and: a)compounds with a general formula 1R′—NH—CHR″—COOR′″  1 where R′, R″ and R′″ equal to or different from oneanother may be H or alkyl groups with the formula C_(n)H_(2n+1) with nbetween 1 and 20, or an aromatic group, or limited to R′, R′″ an ethergroup with the formula (CH₂CH₂O)_(n) with n between 1 and 20 and b)compounds with a general formula 2R″″—CHO  2 where R″″ may be H or an alkyl group with the formulaC_(n)H_(2n+1) with n between 1 and 20, or an ether group with theformula (CH₂CH₂O)_(n) with n between 1 and 20, or an aromatic groupexcept for compounds in which: R′, R″, R′″ and R″″ are equal to oneanother and equal to H; R′ is equal to an ether group with the formula—(CH₂CH₂O)₃—CH₃ and R″, R′″ and R″″ are equal to one another and equalto H; R′ is equal to an alkyl group with the formula —CH₂(CH₂)₆CH₃ andR″, R′″ and R″″ are equal to one another and equal to H; R′ is equal toan ether group with the formula —(CH₂CH₂O)₃—CH₃, R″, R′″ are equal toeach other and equal to H and R″″ is equal to -phenyl-OCH₃; R′ is equalto an ether group with the formula —(CH₂CH₂O)₃—CH₃, R″, R′″ are equal toeach other and equal to H and R″″ is equal to a pyrene group. 10.Functionalized carbon nanotubes obtainable with 1,3-dipolar reaction asclaimed 9 in which said reaction is conducted in a dipolar aproticsolvent chosen from the group constituted by dimethylformamide (DMF),dimethylsulphoxide, sulpholane, orthodichlorobenzene with reagents inexcess and temperature over 100° C. for at least 24 hours.